Rev hang between shifts after IAC restrictor and idle set procedure

Terrorist 5.0

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Hi everyone. After driving the Mustang for a couple months, I decided it was time for an IAC restrictor plate to let the RPM's drop a little faster between shifts, as it was making me slow down traffic sometimes when I actually waited for it. Just a little faster RPM drop would be nice, nothing drastic, especially after I tried driving with the IAC disconnected. I bought the plate, along with a new IAC, as my other one was on its way out for a while. I also wanted to turn the idle up as the previous owner had it set at 500-550 RPM. I put everything together with the small hole facing away from the motor as many people have done and had good results, and went to set the idle. AFAIK there is no agreed on procedure, but I went about it like this:

1. I let the car sit all day so it was nice and cold.

2. I set my throttle stop. The instructions said a 0.010" feeler gauge until it dragged but I didn't have that so two pieces of standard paper sufficed, anyway that plus 1 and 1/2 turns.

3. I started the car cold with the IAC disconnected, and had someone just barely keep it running for 2 minutes.

4. I adjusted the IAC bleed out around 1 and 1/2 turns.

After this the idle was still quite low so I turned the throttle stop another 1/2 turn in like the instructions said, and the idle was good. I shut the car off, plugged the IAC in, started it, and it idled at 650 RPM. Perfect. Shut it off, unhooked the battery, and let it sit overnight.

The next day, I went to cold start it, and the RPM's shot up to 2000, and came down to around 800 in around 7 or 8 seconds. Fair enough, I haven't driven the car yet to judge it. I went for a drive and the RPM hang was awful. Way worse than before. The RPM's even jump up a little now if I don't wait long enough to push the clutch in after letting off the gas. Went home, flipped the plate around, same thing. Tried adjusting the idle down to where it was before, same thing. I set the idle as per the instructions perfectly, and unhooked the battery, and am now waiting for a couple hours for the car to be cold again to see what happens. Any ideas? The car has no vacuum leaks confirmed (smoke tested).

The main things I am looking for is:

1. What is the target TPS voltage when the car is idling? I know the TPS in the SN95 isn't adjustable, but I want to know so I can set my throttle stop accordingly, and I am also going to buy a multimeter to check out what the situation in the sensor is.

2. What is the computer's target idle RPM? AFAIK it is around 650.

3. Which way is the plate supposed to go? It is conflicting everywhere I look.

Thanks everyone.
 

badass98svt

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The IAC restrictor only works temporarily, and then the problem returns.
 
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Terrorist 5.0

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The IAC restrictor only works temporarily, and then the problem returns.
I've heard someone say that one time but they didn't say how long it works for. If I have to reset the computer every once in a while that is ok with me, but for now I need to figure out why it got worse than before. Any ideas?
 

badass98svt

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I've heard someone say that one time but they didn't say how long it works for. If I have to reset the computer every once in a while that is ok with me, but for now I need to figure out why it got worse than before. Any ideas?

Not very long. Only until the ECU figures it out
 

badass98svt

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That is so weird. Is that true with the 4.6 cars too? People there seem to have great success with these things.

Yes, I am referring to the 4.6
I've tried them on a SOHC as well as DOHC. It's just temporary.
 
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Terrorist 5.0

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Yes, I am referring to the 4.6
I've tried them on a SOHC as well as DOHC. It's just temporary.
Ok, well if that’s the case, I have the 5.0, but regardless, what can be done for the rev hang? What do you suggest? I’ve heard of Tweecers and am down to try one but I don’t know how to use it yet. And what is the proper idle setting procedure at least?
 

badass98svt

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You'll have to wait for someone else to chime in on the idle setting. It's been many years since I've done that with a 302
From what I've read I believe it is fixed in a tune.
FYI my tuned Cobra still does it
 
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Terrorist 5.0

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You'll have to wait for someone else to chime in on the idle setting. It's been many years since I've done that with a 302
From what I've read I believe it is fixed in a tune.
FYI my tuned Cobra still does it
Thank you, I did not know that the plate was temporary to that extent. I just need to find the idle procedure then and until I dig into tuning the dash pot out, I’m setting the throttle stop to an acceptable TPS voltage and letting the IAC bleed do the rest for idle. I’m going to unplug the IAC and run it :(
 

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it can be hard to get the perfect size hole. This is an adjustable version.


It is of course just a band aid. I had problems with hanging revs and they went away when I switched throttle bodies but a tune might be needed as well.
 

joemomma

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There is a detailed procedure out there for setting the base idle on the '94-95, it was an article in one of the magazines at one time but there is an online version floating around. Search for "Mustang base idle reset" and you should come across it.

FWIW I have had my restrictor plate installed on my 5.0 for a couple of weeks but I haven't driven the car much since installing it, maybe a tank of gas if that. It certainly helped but didn't correct it 100%. I also did not reset the idle after installing, I should probably do that.
 
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Terrorist 5.0

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Thanks for the ideas everyone, I ended up just disconnecting the IAC and bumping up the idle via the air bleeder. I did mess around with the throttle stop as my original plan was to do the full idle reset, but the threaded rod broke. I rethreaded the hole and replaced the rod with a bolt of similar thread, and set that. My idle is between 600-650, and when cold starting, the car actually idles (most of the time), but you do have to give it a little extra gas to keep it alive when you want to move. Once its all warmed up, it drives very nicely. The hang is basically gone (makes sense), although I think the computer adjusted slightly for the loss of the IAC and may be playing with timing in between shifts. It drops just a little slower than if you were revving it in neutral unless you completely lift off the gas, wait about a second, and then clutch. My final verdict is I wished I did this sooner. Sure losing the IAC kind of blows in some scenarios, but it is not bad at all. More than makes up for it in driving.
 

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